1. Field of the Invention
The present invention relates to an organic electroluminescence device, and more particularly, to an organic electroluminescence device capable of increasing color purity and brightness.
2. Description of the Related Art
Electroluminescence display devices are generally classified into inorganic electroluminescence devices and organic electroluminescence devices depending on the materials used for the emitting layer.
In organic electroluminescence devices, electrons and holes are combined in the emitting layer, to form excitons. Excitons in an excited state move to a ground state and transmit energy to fluorescent molecules in the emitting layer. The fluorescent molecules emit light and thereby form an image.
Organic electroluminescence devices have a higher brightness, require lower voltages, and have faster response times than inorganic electroluminescence devices. Organic electroluminescence devices have been referred to as next generation display devices due to their wide range of colors, wide viewing angle, and excellent contrast.
Generally, an organic electroluminescence device includes an anode layer formed on the surface of a substrate in a predetermined pattern. A hole injecting layer, a hole transporting layer, an emitting layer, and an electron transporting layer are sequentially stacked on the surface of the anode layer. A cathode layer is formed on the surface of the electron transporting layer in a predetermined pattern and is orthogonal to the anode layer. The hole transporting layer, the emitting layer, and the electron transporting layer are organic thin films formed from organic compounds.
In conventional organic electroluminescence devices, maximum light emitting efficiency and brightness is obtained by controlling the thickness of organic thin films. For example, in the electroluminescence device disclosed in Japanese Patent Publication No. hei 4-137485, light emitting efficiency is increased by setting the thickness of an electron transporting layer ranging from 30 nm to 60 nm. The electroluminescence device disclosed in Japanese Patent Publication No. hei 4-328295 increases brightness by making the light emitted from an emitting layer and light reflected by a cathode plate constructively interfere with each other.
FIG. 1 is a schematic diagram of an organic electroluminescence device disclosed in U.S. Pat. No. 6,124,024. Referring to FIG. 1, the total optical thickness is set at a value which enhances the intensity of light having a wavelength of 440–490 nm corresponding to blue light, a wavelength of 500–550 nm corresponding to green light, and a wavelength of 600–650 nm corresponding to red light.
As shown in FIG. 1, the emitted light includes various types of light, such as (i) light emitting from the transparent side of interface B, (ii) light reflected at interface A and then emitting from the transparent side of interface B, and (iii) light reflected at interface B, then reflected at interface A, and then emitting from the transparent side of interface B.
Although U.S. Pat. No. 6,124,024 suggests ranges for the total optical thickness of thin films that allow light to constructively interfere at each wavelength, the ranges of the total optical thickness of thin films are not optimized for both color coordinates and brightness. Moreover, how much the color coordinates and brightness improve in quantitive terms is not known.
Color coordinates and brightness change according to the shape of the spectrum and the wavelength of light. For example, for blue light having a central wavelength of 450 nm, color coordinates improve as the width of a spectrum becomes narrower. Since the shape of a spectrum is usually not symmetrical, even if the widths of spectra are the same, color coordinates improve as the wavelength of light becomes shorther.
Further, the brightness of blue light is degraded as the color coordinates improve. Accordingly, finding the optimal thickness value for the organic layer taking into account color coordinates and brightness is desired. However, the conditions of the sum of thicknesses of the organic layer, the transparent electrode, and the high-refraction layer determined by the formulas suggested in U.S. Pat. No. 6,124,024 do not consider color coordinates and brightness, so it is difficult to find optimal thickness values that optimize color coordinates and brightness for blue light.